The present invention relates to apparatus for programming the operation of heating, cooling and ventilating systems for buildings, particularly buildings which are intermittently occupied, such as office buildings, stores, shopping centers, and other commercial buildings. With the increased cost of energy, there have been great incentives to reduce the amount of energy used in heating and cooling buildings by way of scheduling lower temperatures in the buildings when they are not occupied, sometimes referred to as "night setback systems".
One of the problems encountered in programming building temperatures is that the building must be brought to the desired temperature by the time the occupants arrive at the building for work. If, for example, during the winter the night setback temperature is 55.degree. F. and the building is scheduled for occupancy at 9:00 A.M., the system controller cannot wait until 9:00 A.M. before turning on the heating system because of the substantial time lag between the time the heaters may be turned on and the time the building will reach the desired temperature of occupancy. The same is true when the building is air conditioned and the outside air temperature exceeds the desired temperature of occupancy.
Many systems and techniques have been suggested for controlling system operation during the pre-occupancy period in which the interior of the building and its contents are brought to the desired temperature. One system for achieving the desired temperature at occupancy is referred to as an "optimal start programmer". One such system, referred to as the C-7501 Optimal Start Programmer is sold by Johnson Controls, Inc., of Milwaukee, Wis. This system uses an outdoor sensor for sensing outdoor air temperature, an indoor sensor for sensing indoor space temperature, and a motor-driven potentiometer to generate a simulated "set point" signal which increases from the night setback temperature to the desired occupancy temperature in a predetermined manner.
The length of the preheat period (that is, the period just prior to occupancy during which the simulated set point signal is generated) is determined by the outdoor temperature, as well as other variables, such as the heating mass of the building, the capacity of the heating plant, etc. In general, however, the preheat period is lengthened for colder outdoor air temperatures. When the set point signal becomes equal to or greater than the indoor space temperature, a "plant start" signal is generated to command the heating equipment to begin operation. Thus, the higher the indoor temperature during the preheat period and the higher the outdoor air temperature, the shorter will be the preheat period and the greater the savings in energy.
In these systems which generate a set point signal mechanically, the maximum preheat period is determined by the ratio of a gear box. In order to change the maximum time for the preheat period, a major modification of the programmer is required. Earlier systems which used cam-driven potentiometers to generate the set point signal used the profile of the cam to determine the maximum preheat period.
Because of the long periods required for preheating of buildings (eight hours or longer for a maximum preheat time is not unusual), conventional analog electronic circuit techniques are not used to generate the set point signal because of the prohibitive size of components and lack of accuracy. Further, prior art systems were not able to accommodate both warm up and cool down of the indoor temperature during the pre-occupancy period during which the energy source is actuated, without manually changing a mode switch on the programmer.
The present invention is thus directed to an optimal start programmer in which a set point signal is generated electronically, and preferably using an oscillator circuit which may be synchronized with a line frequency signal and having its output coupled to a counter circuit. A standard timer and controller, which preferably is a controller oriented processor or COP, as it is referred to in the art, also receives the output of the oscillator as a clock signal. The timer generates time and calendar data for determining occupancy periods, non-work days, and the pre-occupancy period during which the temperature of the space is brought from the night setback temperature to the desired temperature at occupancy. The present system is adapted to accommodate both heating and air conditioning, and includes an automatic changeover circuit, (called a mode selection circuit) for determining the operating mode as a function of the mass temperature. For this reason, the preoccupancy period is referred to as a "search" period to indicate that it is generic to both the heating and cooling modes.
The digital counter is enabled at the start of the search period by the timer and controller circuit, and it accumulates counts from the oscillator or timing source.
The output signals of the counter are fed in parallel to a digital to analog converter which generates a staircase or digital ramp signal which is used as a time-variable set point signal during the search period.
The mode control circuit actually defines three modes of operation during the search period. This circuit is responsive to a heating mass signal and a cooling mass signal. Each of these signals, theoretically, is representative of the temperature of the indoor space being controlled, although they may have different readings because of their different locations. They are referred to as temperature "mass" signals because in a dynamic situation, they represent not only the temperature of the space, but the temperature of the mass associated with the space which has an effect on the time required to bring the space to a desired occupancy temperature. Although a single mass temperature signal may be used, in some applications it is preferable to use both a heating mass signal and a cooling mass signal because of the desirability of locating the two sensors in different places.
If the heating mass signal indicates a temperature less than a first predetermined temperature (for example, 65.degree. F.), the mode control circuit defines a heating mode of operation during the search period. If the cooling mass signal indicates that the temperature of the space is greater than a second predetermined temperature higher than the first predetermined temperature (for example, 75.degree. F.), then the mode control circuit defines a cooling mode of operation. The temperature band between the first and second predetermined temperatures defines a range in which the mode control circuit generates an inhibit signal so that neither the heating source nor the cooling source are energized.
When the system is operating in the heating mode, the reference signal to the digital to analog converter which generates the time-variable set point signal is a signal representative of the outdoor air temperature, derived from an exterior sensor. In this manner, the starting point of the time variable set point signal in the heating mode is defined by the outdoor temperature. If the mode control circuit specifies operation in the cooling mode, a fixed voltage signal is used as the reference for digital to analog converter. The output signal of the digital to analog converter is a current, which is then converted to a voltage and subtracted from the reference signal to generate the actual set point signal. The set point signal is then compared with the mass temperature signal and, depending upon the mode of operation, the resulting signal is used to energize the "boilers relay" or the "chillers relay" to energize the heating or cooling source respectively, as conditions require.
Thus, the present invention employs digital circuitry to generate the time variable set point signal and therefore takes advantage of the greater reliability and repeatability of electronic circuitry, particularly in relation to the cam-driven and motor-driven potentiometers of prior systems. By using solid state circuitry exclusively, no moving parts are incorporated in the optimal start programmer whatever. Further, the provision of a mode control circuit prevents inadvertent operation in the wrong mode, for example, as might be caused by failure to set prior systems manually into the desired mode, and it further permits the system to respond to wide changes in outdoor temperature as they are reflected in the mass temperature signal without having to effect a corresponding manual change on the programmer.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.